Purification of xylene-ethylbenzene mixtures



2,834,821 PURIFICATION GF XYLENE-E'I'HYLBENZENE MIXTURES IngolfurBergsteinsson, Orange, Califl, assignor to Union Oil Company ofCalifornia, Los Angeles, Calif., a corporation of California No Drawing.Application June 21, 1954 Serial No, 438,347

9 Claims. (Cl. 260-674) This invention relates to methods for thepurification of mixtures of C-8 aromatic hydrocarbons which containethylbenzene in addition to at least one of the xylene isomers. Morespecifically the invention concerns methods for removing theethylbenzene from the remaining xylenes, whereby the xylenes arerendered more amenable to subsequent separation procedures. Theparticular method employed consists broadly in subjecting the initialmixture of C-8 hydrocarbons to mild isomerizing, or transalkylationconditions in the presence of a fused-ring hydrocarbon acting as analkyl acceptor, e. g. naphthalene and the like. The transalkylationconditions em ployed are carefully selected to avoid 'bothintermolecular and intramolecular isomerization among the xyleneisomers. The conditions are also designed to prevent the migration ofethyl groups from ethylbenzene to the xylenes, whereby the latter may berecoveredsubstantially unchanged. In a specific modification of theprocess an impure mixture containing naphthalene, or methyl naphthalenesand othe aromatics and non-aromatics boiling within the same range, isemployed as the alkyl acceptor, the naphthalenic component thereof beingselectively ethylated thereby providing a. means for saperating the samefrom the remaining aromatics and non-aromatics, as for example bydistillation.

The principal object ofthe invention is to provide an economical methodfor separating ethylbenzene from xylenes. More specifically, theobjective is to provide a specific transalkylation process for effectingthe transfer of ethyl groups selectively to added acceptor molecules inpreference to xylene molecules, under conditions which cause little orno migration of methyl groups. A further object is to provide sepecificaromatic ethyl-group acceptors which are readily alkylated, and henceperimt the use of low temperatures and inexpensive catalysts. A stillfurther object is to provide methods for conveniently separating methylnaphthalenes from narrow boiling range hydrocarbon mixtures whichcontain other aromatics and non-aromatics. Another objective is toprovide economical methods for contacting the C8 hydrocarbons with largemole-excesses of the fused-ring compound. Other objects and advantageswill be apparent to those skilled in the art from the description whichfollows.

Mixtures of aromatic C-8 hydrocarbon isomers are frequently obtainedfrom the destructive distillation of coal tars and from variouspetroleum sources such as reformed gasoline fractions. These mixturesordinarily comprise the four isomers in approximately the thermo- StatesPatent 9 isomers in their pure state. At present pure para-xylene ismost highly desirable as a starting material for producing terephthalicacid. The presence of ethylbenzene greatly complicates the resolution ofthese isomers to obtain pure p-xylene. A convenient method for removingethylbenzene from such mixtures is hence highly desirable.

It is known generally that alkyl groups will migrate between aromaticnuclei under isomerizing conditions. It is also known that the higheralkyl groups, and especially the more highly branched alkyl groups,migrate more readily than the lower and less branched groups. It wouldbe expected therefore that the ethyl group would be thermallydissociated from a benzene ring more readily than methyl groups.However, the ultimate fate of the dissociated ethyl group is acomplicating factor, as is also the fact that it is difficult tomaintain and control conditions which are 100% selective in mobilizingethyl groups but not methyl groups. Dissociated ethyl groups willcombine with neighboring xylene molecules until a thermodynamicequilibrium is reached, producing higherboiling ethyl xylenes. Theultimate yield of xylenes recovered is thereby lessened to the sameextent as the conversion of ethylbenzene to benzene,if no extraneousethyl-group acceptors are present. Moreover, under knowntrans-alkylation conditions, some of the methyl groups from the xylenesare also mobilized, resulting in the formation of methyl-ethyl benzenes,toluene, trimethyl benzenes and the like. The resulting mixtures are"difficult to purify by fractional distillation because of the widevariety of close-boiling isomers and homologs present.

Moreover, without careful selection of an alkyl acceptor, the particularxylene isomer desired may be selectively ethylated. If any xylene isomeris selectively ethylated, the other xylene isomers will not be inthermodynamic equilibrium, and intra-molecular isomerization may takeplace to reestablish an equilibrium xylene composition. Intramolecularisomerization, i. e. the rearrangement of methyl groups on the samebenzene ring, appears to occur more readily than transmigration of ethylgroups between benzene rings; hence regardless of the xylene isomerselectively ethylated by transmigration, the final xylene mixturerecovered may have approximately the same composition as the originalxylenes, but the overall yield will in any case be reduced.

According to the present invention a molar excess, relative to theethylbenze, of a fused-ring compound such as naphthalene is added to theC-8 hydrocarbons. It has been found that the fused-ring compounds aremore readily ethylated by transmigration than are monocyclic aromatics.Hence the selectivity of migration of ethyl groups in preference tomethyl groups may be improved by employing milder conditions, e. g.lower temperatures,

dynamic equilibrium ratios. Their composition may range for examplewithin the following proportions:

For various uses it is desirable to obtain theindividual 7 shorterreaction times, milder catalysts, etc., and relying upon the fused ringcompound to combine rapidly with dissociated ethyl groups, therebyhastening the reaction under mild conditions. Under such conditionssubstantially none of the xylene is ethylated, and the conversion ofethylbenzene to benzene is practically quantitative moreover,substantially no migration of methyl groups occurs. It will be observedalso that the products of transalkylation arevery readily separable bydistillation. The principal products may be benzene (l3. P. C.), xylenes(B. P. l37144 C.), naphthalene (B. P. 218 C.), ethyl naphthalenes (B. P.251-258 C.) and the like. Some higher ethylated naphthalenes may beformed which are not distillable at atmospheric pressure; such productsmay be purified by fractional crystallization or vacuum distillation. i

It has further been found that the mono-alkylated naphthalenes, or othermono-alkyl fused-ring compounds, are still more readily ethylated bytrans-alkylation than the non-alkylated parent compounds. However, thealkyl group on the fused-ring compound should be no longer than theethyl group, otherwise it will tend to migrate to the monocycliccompounds to some extent. Methylsubstituted compounds are preferred suchas a-methyl naphthalene or ,B-methyl' naphthalene, or mixtures there.of. An especially advantageous material consists of certain residualhydrocarbon mixtures, called cycle. oils, resulting from the thermal orcatalytic cracking of petroleum. stocks. Such mixtures are veryrefractory to further cracking operations because of their high aromaticcontent. Their gravity may range between about 14 and 25. API, and theirboiling range between about 205 and 320 C- By fractional distillation, acut may be isolated between about 240 and 250 C. which containsapproximately 50% by weight of and fl-methyl naphthalenes, 20-30% ofother aromatics and 20-3'0% of non-aromatics. By utilizing, this type ofmaterial, the methyl naphthalenes are selectively ethylated, therebyforming high-boiling derivatives which may be readily separated from theremaining aromatics and non-aromatics contained in the original cycleoil. The alltylated naphthalenes recovered may then be utilized as such,or they may be subjected to known dealkylating conditions' to remove theethyl groups, thereby providing substantially pure methyl naphthalenes.Higher boiling fractions from the same type of cracking cycle stock maybe similarly utilized. Such higher boiling fractions may contain forexample anthracene, phenanthreue and methylated derivatives thereof; Thelower-boiling cuts may contain naphthalene.

The present invention embraces the use as alkyl acceptors of purefused-ring aromatic hydrocarbons, or any material comprising substantialquantities thereof. Suitable fused-ring compounds include for examplenaphthalene, phenanthrene, anthracene, chrysene, picene and the like. Apreferred sub-class comprises the mono-methylated derivatives, as wellas in some cases polymethylated derivatives. Ethylated derivatives maybe employed to less advantage. If impure mixtures are employed, thenon-fused-ring components thereof should be of such character as willnot. interfere with. the desired course of reaction. The operativeproportions of fused-ring com.- pounds may range between about 1 and 50moles per mole of ethylbenzene, and preferably between about 5 and 20moles. It is further preferred that the quantity of fused-ring compoundbe in mole-excess of. the total xylene components, thereby furtherinhibiting ethylation of the latter. If the. proportion of fused-ringcome 4 pentoxide may also be employed alone as an adsorbent oxidecatalyst.

The trans-ethylation with the adsorbent oxide type catalysts may becarried out in either the liquid phase or vapor phase. Operativetemperatures range between about 170450 C., and preferably between 190and 300 C. Contact times may vary widely from about 1 minute to 4'hours, depending upon the temperature and the activity of the catalystemployed. Ordinarily the pressures. should be. about atmospheric, but insome instances super-atmospheric pressures or sub-atmospheric pressuresmay be employed. Under conditions of high temperature, or where veryactive catalysts are employed,

it may be desirable to. admix with the feed mixture cerpounds is toosmall, more of the xylene will be ethylated,

while too large a. proportion is uneconomical from the. standpoint ofmaterials handling. costs, and the cost of separation of. the reactionproducts.

The operative catalysts. employed herein may be divided into two generalcategories, each of which permits of somewhat different modes of.operation. In the first category are placedthose isomerization catalystswhich are substantially neutral or only slightly acidic, and are made upprimarily of adsorbent oxide-type materials. having large surface areas.Such materials include for example alumina, silica, magnesia, thoria,zirconia, titania, and the like in the form of activated gels. Mixturesof alumina. and silica are particularly desirable, either of. thesynthetic or natural clay type. A particularly desirable catalystconsists of co-precipitated alumina-silica gel containing between about5% and. 90% of. silica, the re.- mainder being alumina. Various naturalclays such as bauxite, bentonite, pumice, and the like may be employed.The acid-activated bentonite-type clays are. particularly desirable,such for example as are sold in the trade as Filtrol or Superfiltrol'.Such clays may contain from about 30% to 90% by weight of silica, theremainder being largely alumina. This type. ofv catalyst may beaugmented by the addition thereto. of mildly acidic. materials such asphosphoric acid or phosphorus pentoxide, or mixtures thereof. Anhydrousphosphorus tain inert diluents such as steam, nitrogen, carbon dioxideand the like. upon the catalyst, inhibiting destructive crackingreactions and promoting thev transfer of ethyl groups. In vaporphaseprocesses, from about 10% to by volume of inert gases may beemployed if desired.

The second category of operative catalysts comprises the more highlyacidic isomerization catalysts, some of which are known in the art asFriedel-Crafts type catalysts, and some of which are heat-stable mineralacids. Examples of such catalysts include aluminum chloride, aluminumbromide, ferric chloride, zinc chloride, antimony trichloride, hydrogenchloride, hydrogen fluoride, sulfuric acid, concentrated phosphoricacid, boron trifluoride and the like. Mixtures of such materials mayalso be employed, and they may be employed either alone or'supported ona suitable carrier such as silica gel, phosphorus pent'oxid'e, activatedcharcoal, pumice, porcelain chips, clay sherds, etc. The operativereaction conditions for the acidic type catalysts are in general similarto those employed for the adsorbent oxide type catalysts, exceptthat-lower temperature ranges are necessary. With the most active acidiccatalysts, such as aluminum chloride, temperatures. as low as 50 C. maybe employed, while withthe; less active members such as sulfuric acid orzinc chloride, temperatures as high as 260 C. may be employed; Ingeneral, the preferred range for such catalysts lies between about 75and200 C.

The reaction may be carried out either continuously or batch-wise, insingle-stage or multi-stage operation. In continuous operation theliquid or vapor phase reactants may be. passed through a stationary bed,or liquid body of catalyst; or they'may be passed concurrently orcountercurrently to. compact moving beds or liquid streams of catalyst;Alternatively, in vapor phase operation, the well known fluidizedcatalyst type of operation may be employed wherein the catalyst. issuspended under hindered settling conditions inthe flowing reactants. Inbatch operation; the liquid. reactants may simply be digested in contactwith the granular or liquid catalyst. After long periods of operationthe activity of the oxide-type catalysts may decline as a result ofdeposits of gum, carlion and other deactivating materials. Suchdeactivated catalysts may be regenerated. by combusting in a streamofoxygen-containinggas at for example 400 -900" C.

According to another modification of a continuous process, thefused-ring compound may be maintained, in the liquid phase, and the C-8'hydrocarbons may be passed therethrough in vapor phase. The liquid phaseis maintained above" the boiling point of the xylenes at operatingpressure, and may be disposed,v together with the catalyst; in asuitable vessel as a stationary body, or may be continuouslygravitateddownwardly in a bubbleplate column for example while the gaseous xylenesare bubbled counter-currently upwardly. This procedure automaticallyprovides for maintaining a large moleexcess-of fused-ring compound inthe liquid phase. The overhead products may be continuously fractionatedto recover benzene and ethylbenzene-free xylenes, while the bottomsfromthe bubble-plate column may" be continuously fractionated. torecover high-boiling ethylated ma- Steam exerts a modifying actionterials. The catalyst may be stationary in the column, or may movedownwardly with the liquid phase. This type of operation is particularlyadvantageous when the acidic type catalysts are employed, therebypermitting the use of low temperatures which are advantageously abovethe atmospheric boiling point of the xylenes but below that Example I Amixture of C8 hydrocarbons having the following composition by volume:

Percent p-Xylene 20 m-Xylene 48 o-Xylene 15 Ethylbenzene 17 grams of agranular synthetic alumina-silica gelcatalyst containing 80% by weightSiO and 20% by weight of A1 0 The resulting mixture is then subjected tofractional distillation, recovering overhead 112 grams of benzene,followed by 825 grams of mixed xylenes boiling between 138 and 144 C.Analysis of the xylene mixture shows the presence of less than 2% ofethylbenzene. The bottoms product from the distillation is then furtherfractionated to recover 580 grams of naphthalene and 568 grams ofa-methyl naphthalene, showing that the methyl naphthalene was ethylatedin preference to the naphthalene. The remaining still bottoms consist ofmonoand di-ethylated methyl naphthalenes, together with a smallproportion of ethylated naphthalene.

This example demonstrates that over 90% of the ethylbenzene may beconverted to benzene with a 98% recovery of xylenes.

Example 11 Another 1000 gram portion of the above mixture of C-8hydrocarbons is admixed with 3200 grams of a narrow-boiling cut from athermal cracker cycle stock, the cut boiling between 238 and 248 C. Theoriginal cycle oil has a gravity of 24 API and a boiling range of 220 to280 C. Analysis of the 238248 cut showed it to contain about 48% byweight of methyl naphthalenes, 24% of non-aromatics, and 28% ofunidentified aromatic materials. The resulting mixture is then heated inan autoclave as described in Example I in contact with 400 grams ofgranular Filtrol catalyst. The resulting product is then fractionated torecover 97% of the original xylenes, containing less than 1% ofethylbenzene. Upon fractional distillation of the remaining bottomsproduct, 2986 grams of an overhead product boiling be= tween 238 and 248C. is obtained, leaving a residue which is found to boil above 250 C.Analysis of the high boiling residue shows that it consistspredominantly of mono-ethyland di-ethyl-methyl-naphthalenes. Bysubjecting this material to selective dealkylation conditions, a mixtureof aand fi-methyl naphthalenes is obtained predominating in theu-isomer.

This example demonstrates the feasibility of utilizing the hereindescribed invention to not only remove ethylbenzene from xylene stocks,but also to recover effective 1y methyl naphthalenesfrorn complexhigh-boilings'tock's which are likewise difiicult to resolve into theircomponents. The methyl naphthalenes themselves are use ful materials inthe chemical arts as intermediates for dyestuffs, pharmaceuticals andthe like.

Example III This example illustrates the results obtainable bycountercurrent contacting of liquid phase naphthalene with gaseous C-8hydrocarbons in the presence of ferric chloride catalyst.

The ferric chloride catalyst is prepared by pulverizing anhydrous FeClcontaining less than 1% by weight of water. The powdered catalyst isthen suspended in molten naphthalene, employing 15 gms. of catalyst per100 gms. of naphthalene. The resulting slurry is then heated withagitation to 175 C., and trickled downwardly through a bubble-platecolumn containing 12 trays at the rate of'about 5 ml. per minute. Thepreheated, (175 C.) vaporized feed mixture of Example I is passed up-.wardly at a rate suflicient to provide an average vapor liquid contacttime of about 30 minutes. Analysis of the overhead product indicates a99% recovery of xylenes containing about 0.6% ethylbenzene. Theethylated naphthalene-catalyst slurry collected at the bottom of thecolumn is filtered to remove the catalyst, and the liquid isfractionated to recover unreacted naphthalene. The catalyst may then bemixed with fresh or recovered naphthalene and again passed throuugh thecolumn.

The vapor-liquid contacting process of this example is capable of givinga bottoms product which' is 50-70% by volume of ethylated naphthalenes.The trans-ethylation of naphthalene may be carried to this extent byvirtue of the fact that the xylene concentration is very low at alltimes in the liquid phase, and also because no catalyst is present inthe vapor phase. Both of these factors minimize ethylation of xylenes,as well as the reethylation of benzene.

The ferric chloride catalyst of this example, as well as otherFriedel-Crafts type catalysts, may be further activated by the additionof a halogen acid, or small proportions of water, thereby permitting theuse of lower temperatures and/or shorter contact times. An advantageousmethod for activating such catalysts consists in adding small amounts ofgaseous HCl OPHZO to the xylene vapors, e. g. 0.5% to 5% by volume. Inthis manner, the maximum catalytic activity is assured only in zoneswhere ethylbenzene and naphthalene are intimately admixed, andtransalkylation is desired, while a lesser catalytic activity willprevail in zones which are relatively free of one of those reactants,and where transalkylation is therefore not desired.

Substantially similar results are obtained when other acidic catalystsare substituted for FeCl in the above example. Sulfuric acid for examplemay be employed under the same or similar temperature conditions, whilewith aluminum chloride somewhat lower temperatures are preferable e. g.l40l50 C. Shorter contact times I may also be utilized with AlCl e. g.5-15 minutes.

While the above examples are limited to specific conditions andproportions, it is contemplated that those factors may be variedconsiderably to obtain substantially similar results. The foregoingdisclosure is therefore not to be considered as limiting, since manyvariations may be made by those skilled in the art without departingfrom the scope or spirit of the following claims:

I claim:

l. A process for removing ethylbenzene from a mixture thereof with atleast one xylene isomer which comprises subjecting said mixture to atemperature between about and 450 C. in the presence of a catalystconsisting essentially of activated alumina-silica and between about 1to 50moles of a fused-ring aromatic hydrocarbon per mole ofethylbenzene, for a sufiicient length of time to transfer substantiallyall of the ethyl groups from ethylbenzene to said .fusederiughydrocarbon, nd hereaf er recovering su stan ial y ethylbenzeue-ireeXylene.

,2. A process according to claim 1 wherein ,saidfusedring aromatichydrocarbon is substituted with a methyl group.

3. A process according to claim 1 wherein said fusedring aromatichydrocarbon is a methylated naphthalene.

4. A process according to claim 1 wherein said fusedring aromatichydrocarbon is naphthalene.

5. A method for concomitantly removing .ethylbenzene from a firstmixture of C-8 aromatic hydrocarbons and for separating a fused-ringaromatic hydrocarbon from a second mixture of close-boiling hydrocarbonswhich comprises subjecting said first mixture to a temperature betweenabout 170 and 450 C. in the presence of a catalyst consistingessentially of activated alumina-silica, and sufiicient of said secondmixture to provide between about 1 and 50 moles of said fused-ringhydrocarbon per mole of ethylbenzene, continuing the contacting withsaid catalyst for a .sufiicient length of time to transfer a majorportion of the ethyl groups from ethylbenzene to said fusedvringhydrocarbon, and thereafter fractionatice the product to recover asseparate fractions su stantia ly .cthylbenzene-tree Xy e and hyl fu e nghydro arbon- ;6. A process as defined ;in claim 5 wherein Said fusedringaromatic hydrocarbon is substituted with a methyl Bi l- F '7. A processaccording to claim 5 wherein .said fusedring anomatic hydrocarbon is amethylated naphthalene.

,8. A process according to claim 5 wherein said fusedring aromatichydrocarbon is naphthalene.

9. A process as defined in claim 5 wherein said second mixture is a240250 C. .cut obtained from a thermal cracking unit cycle oil.

References Cited in the file of this patent UNITED STATES PATENTS2,385,524 Mattox Sept. 25, 1945 2,411,530 Dreisbach et al. Nov. 26, 19462,447,479 Salt Aug. 17, 1948 2,532,276 Birch et .al. Dec. 5, 19502,656,397 Holzman .et a1 Oct. 20, 1953

1. A PROCESS FOR REMOVING ETHYLBENZENE FROM A MIXTURE THEREOF WITH ATLEAST ONE XYLENE ISOMER WHICH COMPRISES SUBJECTING SAID MIXTURE TO ATEMPERATURE BETWEEN ABOUT 170* AND 450*C. IN THE PRESENCE OF A CATALYSTCONSISTING ESSENTIALLY OF ACTIVATED ALUMINA-SILICA AND BETWEEN ABOUT 1TO 50 MOLES OF A FUSED-RING AROMATIC HYDROCARBON PER MOLE OFETHYLBENZENE, FOR A SUFFICIENT LENGTH OF TIME TO TRANSFER SUBSTANTIALLYALL OF THE ETHYL GROUPS FROM ETHYLBENZENE TO SAID FUSED-RINGHYDROCARBON, AND THEREAFTER RECOVERING SUBSTANTIALLY ETHYLBENZENE-FREEXYLENE.